The present invention relates to a CDMA (Code Division Multiple Access)-based communication apparatus, and more particularly, to an interference signal eliminator that eliminates interference using matrix calculations.
One of conventional methods for eliminating various interference such as interference due to multi-path fading, inter-symbol interference and multiple-access interference, and extracting a demodulated signal is an interference signal elimination method using Joint Detection (hereinafter referred to as xe2x80x9cJDxe2x80x9d). This JD is disclosed in xe2x80x9cZero Forcing and Minimum Mean-Square-Error Equalization for Multiuser Detection in Code-Division Multiple-Access Channelsxe2x80x9d (Klein A., Kaleh G. K., Baier P. W., IEEE Trans. Vehicular Technology, vol.45, pp.276-287, 1996).
An apparatus for implementing the conventional interference signal elimination method using JD will be explained using FIG. 1 and FIG. 2 below. In the following explanation, the number of users to be demodulated is assumed to be n.
FIG. 1 is a block diagram showing a configuration of a conventional interference signal eliminator using JD. FIG. 2 is a schematic diagram showing a frame format used in the conventional interference signal eliminator using JD.
In FIG. 1, a reception signal is sent to delayer 11 and matched filters 12-1 to 12-n. Here, the reception signal above has been received by an antenna (not shown in the figure) and subjected to predetermined processing such as frequency conversion by a radio section (not shown in the figure). In delayer 11, the reception signal is delayed by a predetermined time and sent to multiplication section 14 which will be described later.
In matched filters 12-1 to 12-n, channel estimation is performed for each user using a midamble section (see FIG. 2) in a time slot of the reception signal. That is, in matched filters 12-1 to 12-n, a channel estimated value (matrix) for each user is obtained by finding a correlation between known midambles assigned to users 1 to n and the midamble section of the reception signal above within a maximum estimated delay range. Then, the channel estimated value for each of users 1 to n is sent from matched filters 12-1 to 12-n to Joint Detection (hereinafter referred to as xe2x80x9cJDxe2x80x9d) section 13.
JD section 13 performs the following matrix calculations using the channel estimated value for each user above. That is, a convolution calculation is performed between the channel estimated value for each user and a spreading code assigned to each user and a convolution calculation result (matrix) for each user is thereby obtained.
In this way, a matrix made up of regularly placed convolution calculation results of their respective users (hereinafter referred to as xe2x80x9csystem matrixxe2x80x9d) is obtained. Here, for brevity of explanation, the system matrix is expressed as [A].
Furthermore, the following matrix is obtained by performing a matrix calculation shown in the following expression using the system matrix:
[B]=([A]Hxc2x7[A])xe2x88x921xc2x7[A]H{circle around (1)}
where, [A]H is a conjugate transposed matrix of the system matrix and ([A]Hxc2x7[A])xe2x88x921 is an inverse matrix of [A]Hxc2x7[A].
The matrix [B] obtained from the above matrix calculation is sent to multiplication section 14. In multiplication section 14, data for each user free of interference is obtained by carrying out multiplication processing between the data section (see FIG. 2) of the reception signal sent from delayer 11 and the matrix sent from JD section 13. The data for each user obtained at this time is sent to identifier 15. Identifier 15 performs hard decision on the data for each user sent from multiplication section 14 and demodulated data is obtained.
As shown above, the conventional interference signal eliminator using JD obtains demodulated data with interference eliminated without performing despreading or RAKE combining.
However, the conventional interference signal eliminator using JD has a problem of including not a little possibility that the accuracy of demodulated data will reduce for the reasons described below.
First, the channel estimated values obtained for respective users by matched filters 12-1 to 12-n have the potential for including errors, and therefore the matrix calculation results obtained by JD section 13 also have the potential for including errors. As a result, the accuracy of demodulated data obtained from multiplication section 14 may deteriorate.
Here, errors included in the channel estimated values will be explained with reference to FIG. 3. FIG. 3 is a schematic diagram showing a delay profile of a user obtained by channel estimation in the conventional interference signal eliminator using JD.
As shown in FIG. 3, from a channel estimated value estimated using a matched filter, a user""s path and delay time of the path are obtained. That is, valid paths 31 and 32 with high estimated power are obtained and delay times of the valid paths are also obtained.
In this way, channel estimation is performed for each user. However, the channel estimation results sent to JD section 13 are not only the above mentioned valid paths but include other errors, and therefore the accuracy of the matrix calculation results obtained from JD section 13 deteriorates.
Secondly, in a CDMA- based communication, it is desirable to reduce transmit power of the apparatus on the transmitting side to a necessary minimum in order to suppress interference with other users. Therefore, when there is no data to be sent during a call, a method of sending only the midamble section in the aforementioned time slot (see FIG. 2) is adopted. This method is called xe2x80x9cDTX.xe2x80x9d
In the case where a user (here, suppose user 2) only sends a midamble using DTX, since the conventional interference signal eliminator using JD receives the midamble section from user 2, it recognizes that the data section is also received from user 2. As a result, JD section 13 performs the aforementioned matrix calculations with the understanding that it is receiving the data section from user 2.
However, since user 2 is not sending the data section, the demodulated data resulting from a multiplication between the matrix calculation result from JD section 13 and reception signal includes an error. Moreover, trying to demodulate an originally non-existent signal of user 2 may cause an abnormal operation of the entire equipment.
As described above, the conventional interference signal eliminator using JD has a problem of including the potential for reducing the accuracy of demodulated data obtained.
The present invention has been implemented taking account of the problems described above and it is an object of the present invention to provide an interference signal eliminator capable of extracting demodulated data with high accuracy.
This object will be attained by applying threshold decision to an estimated power value calculated based on the channel estimated value obtained from the reception signal and applying the threshold decision result to matrix calculations.